Home made spectroscope

Friday 4th March 2016

Pretty much everything we can observe about the universe comes from the light we can see from stars. I've been doing an Astronomy course over the last year and I've come to realise just how much information can be obtained from light!

One thing that's really impressed me is how it's possible to find out all kinds of things about the composition of a star from looking at its spectrum. For example helium was observed on the Sun before it was discovered on Earth.

For a few quid I bought some diffraction gratings which smear out an object's light into a wide spectrum.

Making a spectroscope

Apart from the diffraction grating, it's all simple stuff. I bought ten gratings from Amazon for £3 if you'd like me to post you one, and I also have some spare razor blades.

You'll need:

Black electrical tape

Cardboard

A long cardboard tube, about 10cm diameter

A razor blade

A 500 lines/mm diffraction grating

We need to block out almost all the light except for a very thin slit at one end of the tube. This makes for clearer lines in the image.

First of all, cut a circle of carboard that's about the same size as the tube diameter. Cut a rectangle from the middle of the circle that the razor blade will cover. Now carefully break the razor blade in two (obviously it's very sharp) and tape one part to each side of the circle, making sure the sharp bit of the blade protrudes from the semi-circle, like the top right image below.

Now tape the blades onto the card leaving about a 1mm gap between the two halves of the razor blade - like the bottom left image below. A smaller gap gives a clearer image, but needs more light to show a spectrum. 1mm seems to work for me.

Now tape the disc to one end of the cardboard tube, making sure that you still have a 1mm gap when you've finished. When you're done it should look like the bottom right image below. You can check it's okay by looking down the other end of the tube and you should see a very thin slit of light.

Now you need to attach the diffraction grating to the other end of the tube, like this. Make sure that the grating is mounted in the same orientation as the slit, that is when the slit is vertical, the text on the grating is the right way up.

The easiest way to do this is to hold the tube up to a CFL-type bulb in your home and place the grating at one end.

Rotate it around until you can see clear lines in the spectrum - if it's the wrong way round you'll just see a single unbroken line spectrum.

Have a play with it and you'll see what I mean.

Once you've got it in the right orientation, tape the grating to the other end of the tube and you're done! Yes, I taped this one upside down.

So what am I looking at?

Good question. White light isn't one colour, it's a bunch of different wavelengths. When you look through the diffraction grating at a light source, you'll see rainbows off to either side of the slit. The grating spreads the light out into different wavelengths like a prism.

At the blue end (normally depicted on the left, but you might have yours upside down!) is light with a wavelength of 390nm, and at the red end most people can see up to about 700nm. Most lights produce radiation beyond these wavelengths, but you can't see into the ultraviolet or infra-red so it just looks black.

Different light sources display very different spectra. I took some rather beautiful photos so you can see what it looks like.

Incandescent lights

Old school incandescent tungsten bulbs, and halogen lights, produce their light by becoming incredibly hot and emitting radiation as visible light. When you look through a spectroscope at one of these lights, you see a continuous spectrum of light like this:

These kind of lights actually emit more radiation in the infra-red than in the visible part of the spectrum, but my camera and your eyes can't see this, so the red just fades out as the wavelength gets longer on the right side. This is why these lights are really inefficient and also why they're really hot to touch.

Compact fluorescent light

About a decade ago everyone switched to compact fluorescent light (CFL) bulbs in their homes. These bulbs work in a totally different way to incandescent lights. They contain mercury atoms which emit energy in the ultraviolet. We can't see ultraviolet so the inside of the tube is coated in phosphors using various metals including europium and terbium which absorb the UV energy and re-emit it at very specific visible wavelengths.

The mixture is chosen carefully to give a particular colour of light which is how you get warm, cool and daylight CFL bulbs. But unlike incandescent bulbs, they only give off a very specific set of wavelengths of light, as you can see above. With the help of my good friend Wikipedia, you can identify exactly which atoms are emitting which parts of the spectrum shown.

Mercury's pretty toxic so we'd ideally like to move away from CFLs over time. And also as you can see, the light isn't actually that good compared to incandescent bulbs, which is why a lot of people don't like them very much.

LEDs

Enter the Light Emitting Diode. These are lower power still than CFL bulbs, and their spectrum is much more like tungsten bulbs.

Don't be fooled though, LEDs also only emit a very narrow set of wavelengths (I'm not sure I fully understand why) so they are also covered in a layer of phosphors. Most household LED lights gives off a blue light which the phosphors absorb and re-emit as other colours.

Because the mix of phosphors is so clever it gives the appearance of a full spectrum, but you can see a dip in the blue part of the spectrum, and less at the red end than an incandescent light.

The horizontal stripes in the image are due to the shape of the LED torch that I used. Good LEDs produce a very nice colour, use a tiny amount of energy and last a really long time. I'm gradually moving all our lights over to LEDs, starting with the halogen bulbs.

Sunlight

You'd think that the cleanest, most complete spectrum would be sunlight, but that's not the case. The inside of the sun is rather hot, and so it radiates light as a broad spectrum. But as the light passes through the outer parts of the sun, and through the Earth's atmosphere, cooler atoms absorb specific wavelengths of light and the resulting spectrum has characteristic dark bands, which you can clearly see in the image below.

These are called Fraunhofer lines after the person who discovered them, and each one is caused by a specific atom. This is how we know the composition of the Sun, and also how helium was discovered - absorption lines were seen in the Sun's spectrum that didn't correspond to any elements that scientists knew about on the Earth.